And this is what the LSR preview shows. By default, I believe the LSR only
outputs a single line, and so that may be why you see more than we get with
the LSR. It can be set to give a whole page, but then this is a bit bigger
than optimal for this. As I said - I'm just concerned that this may
indicate that the lower marks aren't properly accounted for in the
sizing/positioning algorithms.

The image show why using extra-offset as a workaround is a very bad
idea unless you're only tweaking small elements inside a stave: the
ReheasalMark's bounding box doesn't move.

WOW!

I suppose I should also add that to the LSR snippet. For the kind of
music I engrave the method for extra offset is fine pretty much, and
much less complicated than adding a second voice and then a \layout {}
function.